The Scottish National Party said the PM had "concealed the facts" after the advice, which warns of an "indefinite" backstop deal with the EU, came out.
Ministers were found in contempt of Parliament on Tuesday for providing only a legal overview of the deal.
But the PM said the two documents were consistent and the position was clear.
While the UK would have no unilateral right to withdraw from the backstop - a measure designed to prevent the return of physical checks on the Irish border - she insisted neither the UK nor the EU wanted the backstop to come into force in the first place.
But the Democratic Unionists said the legal viewpoint was "devastating" for Mrs May's prospects of getting the backing of MPs for her deal in a vote on 11 December.
The government argued that attorney general Geoffrey Cox's analysis of the Brexit deal, published on Monday, was adequate and disclosing the full advice would be against the national interest.
Labour and other opposition parties said ministers had "wilfully" refused to comply with a binding vote in the Commons last month which demanded full disclosure and MPs agreed in a vote on Tuesday.